The present invention relates to apparatus and methods for controlling the flow of air through a gas turbine rotor to provide substantially uniform heating and cooling of the rotor discs during transient operation and without cooling air losses during steady state rotor operation.
In modern gas turbines, requirements for high efficiency and output have resulted in significant increases in operating temperatures. This, in turn, has led to the design and construction of composite rotor structures using different materials. It has also led to the development of numerous and complicated internal flow circuits for delivering cooling air to the various portions of the gas turbine, including those exposed to the hot gas, to accommodate operation at increased temperatures.
A major problem in high-efficiency, high-temperature gas turbine operation has been non-uniform heating and cooling of the rotor discs. For example, during transient operating conditions, i.e., start-up and other changes in speed between start-up and the turbine's rated speed, there is a significant temperature differential between the outer peripheral parts of the turbine discs, including the buckets, and the inner portion of the rotor disc. This essentially radial, thermal gradient can cause high thermal stress. Concomitantly during such transient operations, particularly start-up, non-uniformity of heating and cooling in an axial direction also exists, e.g., when the rotor components are heated or cooled only from one side. That is, a thermal gradient exists between opposite sides of the rotor disc in the axial direction. Cooling circuits in rotors frequently provide an air path to the buckets passing over only one side, e.g., the forward side, of the disc but not along its opposite side, e.g., the aft side. Consequently, distortion, for example, dishing, of the disc may occur due to this axial thermal gradient This can lead to changes in the rotor's structural inertia properties, i.e., rotor stability or structural rigidity, with resultant high transient rotor vibration, possible unbalancing and structural failure.
Additional cooling circuits are therefore needed to compensate for the axial thermal gradient during transient conditions and provide substantially uniform heating of the discs. However, additional cooling circuits, when provided consistently and continuously throughout the entire operating range of the turbine, represent significant losses to the efficiency of the turbine. That is, additional cooling circuits are not needed during steady state operation and, if provided, for the needed cooling during transient operations, cause loss of engine efficiency. Consequently, there has developed a need for an air flow control system which minimizes rotor distortion and thermal instability due to non-uniform internal air delivery systems during transient operations and eliminates cooling air losses resulting from those additional cooling circuits during steady state operations. It will be appreciated by those skilled in this art that reference herein to cooling air refers to the compressor discharge air which is quite hot, on the order of 600.degree. F., but which is cool relative to the temperature of the buckets during transient and steady state operations.
In accordance with the present invention, there is provided a linear central actuator for controlling the flow of air through the rotor in a manner to afford substantially uniform heating and cooling of the rotor discs, thereby avoiding thermal instability, stresses and distortion of the discs, while simultaneously avoiding cooling air losses in the system at steady state operation. Thus, the present invention provides such air flow control in a manner which introduces the cooling air to the rotor discs to afford uniformity of heating only during the times necessary to do so, i.e., transient operations, including start-up, whereas during steady state operation, the additional cooling circuit is automatically shut down to avoid cooling air losses. To accomplish the foregoing, there is provided, in accordance with the present invention, a linear actuator for controlling flow of air, particular compressor extraction air, to the rotor discs, including a pair of generally cylindrical actuators having opposite ends secured respectively to flanges at the opposite ends of a rotor shaft mounting a plurality of rotor discs and spacers. The actuators are disposed concentrically about the rotor axis. The distal or interior ends of the cylindrical actuators overlap and lie concentric one with the other. Each actuator is provided with a plurality of openings, preferably both circumferentially and axially spaced one from the other, for passing air from the compressor to opposite sides of one or more discs, preferably the aft rotor disc or discs to effect uniform heating or cooling thereof depending upon the application. More particularly, the actuators are each responsive to temperature changes to thermally expand in the axial direction. The alignment or misalignment of the openings within the actuators is thereby controlled by the thermal expansion of one or both of the actuators in the axial direction. Thus, when the openings in the overlapped portions of the actuators are misaligned, air cannot flow through the openings. When the openings are partially or fully aligned, air may flow therethrough to the opposite sides of the rotor disc or discs. That is, the degree or extent of registration of the openings in the overlapped portions of the respective actuators is determined by the thermal expansion of the actuators. Thus automatic control of the flow of air through the openings and hence, for example, heating the interior portions of the rotor disc is provided.
At start-up, i.e., when the rotor is cold, the openings through the actuators are initially misaligned. Upon start-up, compressor extraction air is ducted internally through cooling passages to supply air to the first and second-stage buckets along their rear and front sides, respectively. These cooling air passages extend past the root of the first-stage disc and consequently provides flow of air over the outside of the first actuator and through its non-overlapped openings into the interior thereof. As the compressor extraction air heats the initially cold first actuator, it thermally expands axially to displace its openings in a rearward axial direction. Depending on factors such as the time constant and coefficient of thermal expansion, the openings of the thermally expanding first actuator begin to overlap the openings in the second actuator at one or more axial locations. This, in turn, permits compressor extraction air from within the overlapped actuators to flow through the registering openings and radially outwardly into chambers on the opposite sides of the aft rotor disc or discs. As air begins to flow through the aligned openings, that air, in turn, heats the second actuator causing it to thermally expand in an axially opposite direction with respect to the direction of expansion of the first actuator, i.e., an upstream or forward direction. This additional expansion of the second actuator increases the aggregate area of the actuators in registry one with the other, hence increasing the air flow entering the chambers on opposite sides of the aft rotor disc or discs and affording uniform heating thereof.
The materials and geometry of the actuators are chosen such that maximum registration of the openings through the actuators is obtained shortly before the turbine obtains a steady state temperature. As the rotor temperature increases to its steady state, the rotor structure continuously heats up. This results in expansion of the rotor which, in turn, displaces the second actuator in an axially rearward direction. This displacement moves the openings of the second actuator in a direction decreasing the aggregate area of the registering openings and hence decreasing the flow of air through the openings. At a predetermined time after start-up or other transient operations, and upon obtaining steady state operations, the rotor displacement is such that the openings are totally misaligned whereby air flow through the openings is completely choked off. That is, when the rotor has obtained its steady state temperature, the additional air circuit affording uniform heating of the aft rotor on its opposite sides is closed off, hence avoiding cooling air losses.
It will be appreciated that the response of the actuators and, hence, the movement of the openings into aligned, partially aligned or wholly misaligned conditions is dependent upon a number of factors, including the diameters of the actuators, the size, number and shape of the openings in both actuators, the choice of actuator materials, i.e., their coefficients of expansion and conductivity, the structural material forming the rotor discs, the time constants of the actuators and rotor discs, the actuator lengths and the cooling air flow and pressures.
Significant advantages reside in the foregoing-described apparatus and method of operation. For example, major components of the gas turbine rotor assembly can be heated quickly and uniformly, thereby reducing stresses, weight and material costs. A thermally stable rotor structure is provided, with no loss in rotor inertia (no thermally induced vibration). There are no losses of cooling air at steady state operation because cooling air is used to afford uniformity of heating in the rotor discs only during transient operations, including start-up. The system is self-regulating by a simple linear motion in an axial direction. For some rotor materials, preheating the bores of the discs early in the transient condition enables providing discs formed smaller in size than without preheating, a desirable feature from rotor life, cost and producibility standpoints. Design flexibility is also afforded by providing a capability to adapt the components to a combination of flow areas. Additionally, the parts are self-contained in a low "g" environment, i.e., a low stress environment adjacent the rotor axis. The actuators are accessible from the rear of the gas turbine for service and do not require the turbine to be opened for service. The actuators can be readily modified to adjust flow rates and shift time response curves when operating conditions change. Finally, transient bore heating of turbine discs is accomplished without compromising bucket supply pressures. Also, as a further embodiment hereof, the actuators may be modified to control bucket cooling flows during transient or steady state operations.
In a preferred embodiment according to the present invention, there is provided a gas turbine rotor assembly, comprising a rotatable shaft, a plurality of turbine rotors each including a disc mounted on the shaft and turbine buckets on the discs along their outer rims. A pair of cylindrical actuators has opposite ends thereof secured respectively to the shaft and adjoining ends free and overlapping concentrically one within the other radially inwardly of the discs. At least one of the actuators is responsive to a change in temperature to expand in one axial direction relative to the other of the actuators, the actuators having at least one opening each therethrough and in the overlapping portions. Means are provided for supplying compressor extraction air within the cylindrical actuators for communication through the openings, one actuator being movable in one axial direction in response to a change in temperature during transient turbine operation to register at least in part its opening with the opening of the other actuator to enable air to flow from within the actuators through the registered openings to opposite sides of one of the rotor discs.
In a further preferred embodiment according to the present invention, there is provided a gas turbine rotor assembly, comprising a rotatable shaft, a plurality of turbine rotors each including a disc mounted on the shaft and turbine buckets on the discs along their outer rims. A pair of cylindrical actuators has opposite ends thereof secured respectively to the shaft and adjoining ends free and overlapping concentrically one within the other radially inwardly of the discs, at least one of the actuators being responsive to a change in temperature to expand in one axial direction relative to the other of the actuators, the actuators having at least one opening each therethrough and in the overlapping portions, the openings at least partially registering one with the other. Means are provided for supplying compressor extraction air within the cylindrical actuators for communication through the registering openings, the one actuator being movable in one axial direction in response to a change in temperature during transient turbine operation to change the extent of registration of the openings relative to one another thereby to alter the flow of air from within the actuators through the registering openings to opposite sides of one of the rotor discs.
In a further preferred embodiment according to the present invention, there is provided a method of operating a gas turbine rotor assembly having a rotatable shaft, a plurality of turbine rotors mounted on the shaft, each including a disc with buckets along its outer rim, and a pair of cylindrical actuators defining an air channel and overlapping portions with openings therethrough for supplying air to the rotors, comprising the steps of (a) thermally expanding one of the actuators in one axial direction to register at least part of the openings through one actuator with the openings through the other actuator to enable flow of air from the channel to at least one rotor and (b) thermally expanding the other of the actuators in an axial direction to change the extent of registration of the openings one with the other and alter the flow of air from the channel through the registering openings to the rotor.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for uniformly heating during transient operation, including start-up, opposite sides of one or more discs of a turbine rotor and without cooling air losses during steady state operation.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.